1. Field of the Invention
The invention relates to a method and an apparatus for wave and/or vapor phase soldering of electronic components.
The invention may be used in the fabrication of electronic components in which soldering has nowadays to be performed in through-hole mounting on conventional components as well as on surface mount devices (SMD). Another field of application of the invention is soft-soldering of metallic semi-fabricated components.
2. State of the Art
It is well known that for soldering surface mount devices, reflow soldering processes are used. In such processes, the desired solder connections are fabricated by solder being reflowed and remelted as a solder paste by the application of energy. In known reflow soldering processes infrared radiation (infrared soldering) or hot gas (convection soldering), more particularly nitrogen, or laser radiation (laser soldering) or saturated vapor as in vapor phase soldering, are used. The most common processes utilized nowadays are infrared soldering, convection soldering and vapor phase soldering. Whereas infrared soldering is primarily used in connection with components of average complexity, low product variety and high volume, vapor phase soldering is particularly well suited for a high throughput or high volume at small spacings and high package densities, and it is for this reason used in SMD technology. With the development of new vapor phase soldering processes without fluorohydrocarbons as a secondary medium, these soldering processes have gained the greatest significance among the reflow soldering processes, particularly in view of the still more complex SMD components.
Conventional vapor phase soldering processes have been described, for instance, in German patent No. 3,518,444, European patent No. 0.349,094 and in the paper Produktronik, 1/2, 1993, pages 28-32, "Dampfphasenlo tanlagen in neuer Technik, Zum Leben erweckt" (Vapor Phase Soldering Equipment in New Technology, Resurrected) by H. Beine, in which the products or components to be heated are moved completely into the vapor range and are heated completely to the melting temperature of the medium used. In this connection, see in particular the operations diagram of FIG. 2 of the last mentioned paper in which the vapor line rises above the product to be soldered as soon as the solder material is heated to the boiling temperature of the used medium. In vapor phase soldering energy is released as heat on the surfaces of the solder material when a saturated vapor phase condenses. The solder reflows because of the released condensation heat, wets the components to be joined and leads to a material connection between the component connector and the associated contact structure on the component support. The thickness of the condensation film at or on the component to be connected determines the soldering time on the basis of the surface resistance. The vapor condensate also acts as a cleaning agent as it rinses dirt and other sediments off the component (contacts). The saturated vapor phase is generated by a boiling primary fluid. Thus, the boiling point of the primary fluid determines the operating temperature of the soldering process, which remains constant relative to the process. In principle, a thermal overload of the component is avoidable by the components being heat resistant at least up to the soldering temperature. The selection of a primary fluid scaled to the boiling temperature serves to process different solder material.
The characteristics of a vapor phase soldering process are precisely defined soldering or operating temperature, inert atmosphere, uniform heating of the soldering goods, and it is suited particularly for processing components of very different heat capacities.
Its disadvantage is that aside from SMD components (more particularly tantalum capacitors) it does not also possible with reasonable effort to solder through-hole mounting components. Moreover, selective heating is not possible, i.e. the individual components must all be temperature resistant up to the soldering temperature, which either limits the soldering temperature or the solder material, or requires retrofitting of those components which are not temperature resistant. This affects particularly electrolytic capacitors which as a rule are designed for operating temperatures up to about 100.degree. C., but which outgas at higher temperatures, and are destroyed. Other affected components are those containing non-temperature resistant plastics such as plugs and switches.
A further disadvantage is the fact that soldering temperatures have to be chosen which lie about 50.degree. C. above the melting temperature, for instance 183.degree. C. in the case of eutectic lead-tin solder in order to yield the best possible wetting and spreading of the solder.
A wave soldering process (in brief: wave soldering) is generally known for soldering components, especially of the through-hole technology. In this process, the lower surface of a component support carrying components on its upper surface is put into contact with a liquid solder material so that the contact legs protruding through the component support are wetted by solder and a soldered connection is formed after removal of the support from the solder. The solder material is more particularly provided as a wave of solder, the solder being pumped out of the solder bath forming an arch and is retrieved again and returned to the solder bath. If necessary, any impurities are removed as well. In this process, electrodynamic solder pumps are far superior to mechanical solder pumps.
Wave solder processes for soldering electronic components with eutectic lead-tin solder (melting temperature 183.degree. C.) operate, depending upon the flux used, at a preheat temperature of about 110.degree. C. to 130.degree. C. at the side of the components or upper surface of the component support which at the opposite surface of the component support results in maximum temperatures of about 160.degree. C. to 180.degree. C. The energy additionally required to reach a soldering temperature of about 200.degree. C. to 210.degree. C. at the component surface is furnished by the solder wave from the lower surface of the component support. To this end, the solder wave temperature is set about 50.degree. C. higher than the soldering temperatures. Solder wave temperature between about 245.degree. C. and 260.degree. C. are usual. In general, in known wave solder processes the solder wave temperature may be set up to about 270.degree. C. For solder heat resistance of circuit boards the American standard J-STD-001 defines a temperature of 288.degree. C. maximum.
The disadvantage of conventional wave soldering processes is that they usually operate in ambient atmosphere so that upon separation of the solder wave from the lower surface of the component support, bridges and burrs will form which permit soldering at spacings of up to about 0.5 mm only. In addition, the solder wave has to be of a temperature which exceeds the soldering temperature by about 50.degree. C. so that following the transfer of energy from the solder wave through the lower surface of a component support to the upper surface the solder temperature there is safely reached. The problems of temperature sensitive components and the necessity of complex retrofitting of such components apply equally in this context. Further limitations result from the use of lead-free solders the melting temperatures of which lie significantly above the melting temperatures of 183.degree. C. of eutectic lead-tin solders often used at present. Furthermore, either oil or nitrogen are often used to cover the solder bath.
A process and an apparatus for soldering electronic components are known from German published patent specification DE-AS 2,442,180. More particularly, FIG. 3 of this publication discloses a process suitable for a wave soldering process wherein circuit boards provided with components are moved through a container. A solder wave or fountain is formed within the container in a known manner. In addition, the container contains a primary liquid heated to its boiling point by heating elements, and which vaporizes. The saturated vapor permeates the entire container and is prevented by cooling devices from escaping at the entrance and exit channels of the container serving to feed the components into and out of the container, thereby to minimize losses of primary liquid. In this process, the wave of molten solder material is kept at a temperature above the boiling point of the primary liquid which, in turn, lies above the melting point of the solder.
Components to be soldered by a wave soldering process are fed into the container at room temperature through an input channel. They are preheated to a temperature above the melting point of the solder by the latent evaporation heat of the hot saturated vapor. As a result of the preheating, any solder layer on the components is melting and thus affects melting of the solder. The components enter the wave of molten solder in a preheated state, and are soldered therein. Rather than heating the components, the solder wave serves only to apply the solder. Because of this, the application of heat and the application of solder constitute two separate easily controllable operational steps. As the components leave the wave, the solder present on the components including the solder applied by the wave remain in a molten state so that because of solder flow resulting from surface tension soldering defects are prevented. Only after exiting from the channel does solidification of the solder take place as a result of cooling down to room temperature.
In a further embodiment of the known process, a primary liquid is chosen having a boiling point below the melting point of the solder. In that manner, a component is preheated by the vapor only, and the remaining energy for attaining the soldering temperature as well as the solder material are supplied by the solder wave.
The drawback of the invention described in German published patent specification DE-AS 2,442,180 is that temperature sensitive components have to be retrofitted because of the non-selectivity of the soldering process. This is especially disadvantageous in mass production because it necessitates additional process steps and because selective retrofitting of individual components is involved and often cannot be performed with the necessary quality.